Sprinkler systems of this kind usually having a steering unit and several successively connected outriggers, whereby the steering unit is mostly laterally arranged and serves for steering correction, i.e. the correction of a deviation from the guideway, while the outriggers adjust their mutual advance via a directional steering.
A linearly movable irrigation system is known from the German open application No. 2 851 425, which follows via a sensor arrangement an underground electrical conduit path, and is provided with means to stop the propulsion of the outrigger which is the most extreme at the moment, in case of deviation from the path. The sensor arrangement is mounted on a horizontal bar, which in turn is rigidly connected with the outrigger, preferably at an angle of 90.degree.. The reference point determining the position of the sprinkling system is fixed from the point of view of its position with respect to the sprinkler pipe.
Further, it is proposed to support the bar at the outrigger arm at the irrigation tower so that it is rotatable about a vertical axis, for an oscillating motion and to slidably guide its free end along a guiding means arranged at the margin of the field (rail, cable, etc.). The bar's angle of rotation measures the deviation. An angle sensor located in the area of the axis of rotation detects each deviation from the right angle and transmits the steering order to the irrigation tower which is the most extreme at the moment. These two steering modes presume that the outrigger or the irrigation tower to which the bar is mounted, deviates. This deviation is especially critical when this outrigger is the one with the steering unit and the thereon mounted heavy equipment and is at the same time, the most extreme element of the irrigation system.
Depending on the direction of travel and the kind of deviation reading of the sensor mechanism, one of the two most extreme irrigation towers is immobilized and becomes thereby the rotation point for the continuously moving irrigation system, which can execute a turning motion. In the course of this turning motion, the error (twisting or lateral deviation) is compensated and the completion of the correction is again detected by the sensing device, whereafter the immobilized tower receives the order to continue its travel. It is possible that in the case of gross deviations of the actual travel path from the desired one it is necessary to perform several successive corrections, until the correction operation is concluded.
The mentioned outrigger is used as a stationary steering tower of a pivot-irrigation system, based on the steering characteristics shown in the German open application No. 3 049 436; the whole system revolves about this tower until the orientation has been again set on its path corrected. Due to the heavy weight, the wheels of the steering unit sink into the softened ground during the sprinkling operation and produce ruts, which after the conclusion of the sprinkling operation dry and harden. When the system travels back to its starting position at the field's end, without sprinkling, the wheels move in the formed rut, cannot deviate laterally and travel like on tracks. As a result, the steering unit cannot be effective, which leads to the fact that the forerunning irrigation tower cannot receive a stop signal. Under these circumstances it can happen that all outrigger elements continue their travel, until a so-called "banana" forms, which means for the steering unit that the pipe system together with the frame can be dangerously bent between the two irrigation towers. This bending leads to damage or to the breakage of the installation.
The Canadian patent No. 1 135 301 shows a linearly movable irrigation system, which is equipped with a special directional-steering installation, permitting the mutual adjustment of the individual outrigger elements. Thereby, the motion for the directional steering mounted on the outrigger at the tower side is taken over by a couple of cables from the opposite end of the outrigger. The rigid square formed this way by the pair of cables and the two cross beams defines the position of the outrigger, so that errors due to bending of the same which would be picked up by the directional steering, can be avoided. The polygon is involved in the correction of the advance movement of the irrigation system in order to twist the two switch boxes arranged between two certain irrigation towers--which are designated for the correction of the advancement--in such a manner that an angling to the neighboring pipe segment in the case of a deviation is apparently minimized or increased, which leads to an immobilization of one of the two towers, and to a pivoting of the system thereabout or to a restart of the tower. For the detection of a lateral displacement of the system, an additional switch box with sensor means is provided.